1. Field of the Invention
The present invention relates to automatic compensation for the zero offset of an analog-digital converter used notably in a radar reception system with digital demodulation.
2. Field of the Prior Art
In a standard radar reception system, it is the usual practice to change to digital mode only at the demodulator outputs that give the components, in phase and quadrature, of the video signal. There are then two analog-digital converters parallel-connected to the outputs, in phase and in quadrature, of the demodulator in quadrature. The analog-digital converters as well as the demodulator in quadrature, which is made in analog form, have imperfections which make them generate parasitic d.c. components that evolve as a function of the variations in different parameters such as the temperature, the supply voltages and the frequency of the transmission carrier. It is the usual practice to remove these parasitic d.c. components by isolating them in the radar noise through a measurement of the mean values of the amplitudes of the output signals of the two analog-digital converters during a radar silence, this measurement being made for example on the furthest range gates reached after each transmission of an interrogation pulse for which the return of a significant echo is improbable, and by deducting these mean values from the output signals of the analog-digital converters.
A system such as this has at least two drawbacks: firstly, it calls for special digital circuits with a complex structure to carry out measurements of mean values on the noise of the radar at the output of the analog-digital converters and, secondly, it makes it necessary to have radar silence during the measurement period following the transmission of each interrogation pulse. This reduces the operational time of the radar and may be impossible to obtain in the presence of a jammer.
With the advances made in terms of the frequency of the analog-digital converters, it is possible, in a radar reception line, to replace the two analog-digital converters that work in the video band at the outputs of the demodulator in quadrature by a single analog-digital converter that works in intermediate frequencies and is placed at the input of the demodulator in quadrature which is itself made in digital form. In this case, the imperfections that arise from the analog type construction of the demodulator in quadrature are removed but, in the spectra of the video signal components in phase or in quadrature that are available at the output of the demodulator, there still remain parasitic d.c. components due to the zero offset of the analog/digital converter.
As in the foregoing case, it is possible to eliminate these parasitic d.c. lines by detecting their amplitudes through the computation of the mean values of the radar noise at the outputs, in phase and in quadrature, of the demodulator in quadrature during a radar silence, and by deducting them from the output signals, in phase and in quadrature, of the demodulator. However, the same drawbacks as above are found again, namely the need for digital circuits with a complex structure to take the mean values on the radar noise and the necessity of having periods of radar silence to make the measurements.